1. Field of the Invention
The present invention relates to an organic electro-luminescent display apparatus. More particularly, it is preferable for an organic electro-luminescent display apparatus provided with a polycrystalline-Si TFT structure having excellent characteristics.
2. Description of the Related Art
In accompaniment with the arrival of full-fledged multimedia age, attention has been focused on organic electro-luminescent display apparatus as a next-generation flat-plane-type display apparatus which are expected to replace the conventional liquid-crystal display apparatus. The organic electro-luminescent display apparatus exhibits excellent characteristics such as spontaneous light, wide field-of-view angle, and high-speed response characteristics. In the so-called bottom-emission-type organic electro-luminescent display apparatus, which is one type of the organic electro-luminescent display apparatuses, a large number of organic electro-luminescent elements are arranged in a matrix-like configuration on an insulation substrate the representative of which is a glass substrate (which, hereinafter, will be referred to as “glass substrate”). Here, each organic electro-luminescent element is configured in such a manner that a transparent lower electrode such as ITO, an organic layer including hole transport layer, electro-luminescent layer, and electron transport layer, and a low-work-function upper electrode (i.e., reflection electrode) are formed in this sequence. Moreover, when an about a-few-volts voltage is applied between the lower electrode and the upper electrode, holes and electrons are injected into the upper electrode and the lower electrode respectively. Then, each hole and each electron are recombined with each other within the electro-luminescent layer via the transport layers, thereby resulting in generation of an exciton. This exciton emits light when it returns to the ground state. Furthermore, this emitted light passes through the lower electrode having transparent property, then being extracted from the rear surface side of the substrate.
Also, in the so-called top-emission-type organic electro-luminescent display apparatus, which is another type of the organic electro-luminescent display apparatus, a large number of organic electro-luminescent elements are arranged in a matrix-like configuration on the glass substrate. Here, each organic electro-luminescent element is configured in such a manner that, contrary to the above-described bottom-emission-type display apparatus, a low-work-function lower electrode (i.e., reflection electrode), the organic layer including hole transport layer, electro-luminescent layer, and electron transport layer, and a transparent upper electrode such as ITO are formed in this sequence. Emitted light generated within the electro-luminescent layer is extracted after passing through the upper electrode which is positioned on the opposite side to the glass substrate.
The organic electro-luminescent display apparatus, where the organic electro-luminescent elements are used as the pixels, are classified into the simple-matrix type and the active-matrix type, depending on driving schemes for the pixels. In the simple-matrix-type organic electro-luminescent display apparatus, the organic layers, each of which includes the hole transport layer, the electro-luminescent layer, and the electron transport layer, are formed at positions at which a plurality of anode lines and a plurality of cathode lines intersect with each other. Each pixel is lit up only within a selected time during a one-frame time-period. This selected time becomes equal to time width calculated by dividing the one-frame time-period by the number of the anode lines. The simple-matrix-type organic electro-luminescent display apparatus possesses an advantage that its structure is simple. In the simple-matrix-type display apparatus, however, the selected time becomes shorter as the number of the pixels becomes larger. Accordingly, it becomes necessary that the average luminance during the one-frame time-period be made equal to a predetermined value by increasing driving voltage and thereby increasing the instantaneous luminance during the selected time. On account of this necessity, life time of each organic electro-luminescent element becomes shorter. Also, since each organic electro-luminescent element is driven by current, a voltage drop due to line resistance occurs in the case of, in particular, a large screen. Consequently, applying the voltage to each pixel uniformly becomes difficult, and as a result, a luminance variation occurs within the display apparatus. On account of the drawbacks described above, there exists a limit to the high-definition and large-screen implementation of the simple-matrix-type organic electro-luminescent display apparatus.
Meanwhile, in the active-matrix-type organic electro-luminescent display apparatus, each of driving elements, which are configured with switching elements including a plurality of thin film transistors (TFTs) and a plurality of capacitors, is connected to each organic electro-luminescent element which configures each pixel. This configuration allows each pixel to be lit up during the entire one-frame time-period. On account of this configuration, there is no need of the necessity for increasing the luminance. This makes it possible to enhance the life time of each organic electro-luminescent element. Accordingly, it is considered that the active-matrix-type organic electro-luminescent display apparatus is advantageous for the high-definition and large-screen implementation of the display apparatus. Each TFT for driving each organic electro-luminescent element is requested to exhibit characteristics that mobility is high and that a threshold-value (Vth) shift is small. Usually, the coplanar-type polycrystalline-Si TFT where polycrystalline Si having excellent-characteristics is applied to the semiconductor layer is used instead of the stagger-type amorphous-Si TFT used in the liquid-crystal display apparatus and the like.